Walkthrough
After completing the Main Story and obtaining the National Pokedex from Professor Oak, the Legendary Pokemon known as Raikou will begin to randomly roam throughout the Kanto Region if you chose Squirtle as your Starter Pokemon when your journey first began. Raikou is one piece of the Johto Legendary Beast Trio, being a pure Electric Type Pokemon. If found, Raikou will be at Level 50 and will know the moves Thundershock, Roar, Quick Attack, and Spark.
Fortunately, Raikou will attempt to flee on the first turn of the battle with it. You can attempt to from prevent it from doing so by having your Pokemon use a move known as Block, but there's a risk in doing that due to a known glitch that will prevent you from ever finding Raikou again if it uses the move Roar after being blocked from fleeing the battle. Because of this, it would be extremely wise to use your Master Ball on Raikou if it's the Legendary Beast found in your journey.